A curable composition containing an oxyalkylene polymer having silicon-containing functional groups crosslinkable through the formation of siloxane bonds (hereinafter, simply referred to as “reactive silicon groups”) and, optionally, an alkyl (meth)acrylate polymer containing reactive silicon groups is disclosed in, for example, Japanese Unexamined Patent Application Publication Nos. 59-122541, 60-31556, 63-112642, and 6-172631.
Among these publications, Japanese Unexamined Patent Application Publication No. 63-112642 discloses a composition containing a copolymer containing long-chain alkyl (meth)acrylate. This composition has various practical characteristics, such as transparency and tensile properties, superior to those of other compositions disclosed in the other publications described above. However, long-chain alkyl (meth)acrylate monomers, i.e., the starting material of the copolymer containing long-chain alkyl (meth)acrylate, are generally expensive and are difficult to handle since long-chain alkyl (meth)acrylate monomers are solid at room temperature. Moreover, in a typical process used to synthesize a copolymer containing alkyl (meth)acrylate monomeric units by polymerization, the monomeric unit must be cooled in advance for the safety reasons. Long-chain alkyl (meth)acrylate monomers tend to precipitate when mixed with other monomeric units cooled in advance, which is a problem. In general, alkyl (meth)acrylate polymers exhibit a higher glass transition temperature, larger cohesive force, and higher viscosity than those of oxyalkylene polymers. Thus, the viscosity of a composition containing an oxyalkylene polymer and an alkyl (meth)acrylate polymer increases, which is problem from a practical viewpoint.